Pleistocene environments and human presence in the middle Atbara valley (Khashm El Girba, Eastern Sudan)

A 50m thick Pleistocene fluvial succession is extensively exposed in the karib (badlands) area along the Atbara river from Khashm El Girba to Halfa Al Jadida. Through a widespread major unconformity this succession has been subdivided into the Butana Bridge Synthem (BBS) and Khashm El Girba Synthem (KGS). In the latter minor unconformities mark the boundaries between KGS1, KGS2 and KGS3 subsynthems. The BBS is 10m thick, starts with braided stream gravel and terminates with high-sinuosity river sand. An intermediate silty interval with a well-developed calcrete marks a period of reduced clastic input and morphological stability. The BBS yielded vertebrate remains and many Acheulean artefacts and was deposited from the late Early Pleistocene to the early Middle Pleistocene. After a gap of some hundred thousand years the sedimentary record continues with 40m thick KGS fluvial deposits. They are quite diversified and include sands from meandering rivers (KGS1) abruptly interrupted by braided river deposits that evolve to sinuous river sands (KGS2), and, finally, from braided river pebbly sands to sheet flows (KGS3). The KGS yielded abundant vertebrate remains and late Acheulean to Middle Stone Age artefacts. Mollusc patch reefs with stromatolitic coatings at the base of the KGS2 and KGS3 gave U/Th ages of 126.1kyr+/−1.0kyr and 92.2kyr+/−0.7kyr, respectively. These datings, together with fossil assemblages, and artefacts indicate a late Middle Pleistocene to Late Pleistocene age for the KGS. The entire succession makes a northward (Goz Regeb area) transition into fluvio-lacustrine deposits related to the “Atbara palaeolake”. Through palaeohydrological analyses bankfull discharges have been estimated in some KGS1 and KGS2 fluvial channels. They resulted into one order of magnitude less than the present-day Atbara river. The variations in fluvial style and discharge were connected with climatic changes, river network modifications induced by tectonics, and palaeolake Atbara level variations. Due to defined time constrains, the climate changes recorded in the KGS are matched with the Pleistocene Marine Isotope Stages (MIS). The KGS1 meandering rivers can be referred to the MIS 7 wet period, and the episodes of increasing rainfall in KGS2 and KGS3 to the Eemian MIS 5.5 and MIS 5.3 followed by arid conditions (MIS 4?). Despite fossils and facies indicate environmental changes from arid savannah during the BBS to grassland with water pools during the KGS, the Atbara valley was always favourable to human settlement. Our study allowed to reconstruct, although discontinuously, the environments and the occurrence of human presence from the late Early Pleistocene to the Holocene. During this period hominins on their way from East Africa to Eurasia found abundant faunas and more or less perennial streams in the Atbara valley.